Within this BAG proposal we aim to investigate the structure-function relationships of various soluble and membrane proteins in ground and intermediate states that are involved in signal transduction processes and/or enzymatic substrate conversion. In photosensitive proteins such as phytochromes and DNA photolyases we want to visualize in detail the different intermediates of their photocycle or changes after defined light exposure and thereby improve the understanding of their signalling mechanisms. Light-activated rhodopsins are found in all three domains of life and serve as a link to convert solar energy into biological functions such as signal transduction, ion transport and modulation of enzyme activity Here we investigate wild-type and mutant variants of eukaryotic and microbial rhodopsins that function as ion/proton pumps at the structural level. In addition, we analyse different hydrogen-converting oxygen-tolerant [NiFe]-hydrogenases under specific environmental conditions.